1. Field of the Invention
The present invention relates to an electrophotographic photosensitive member, and a process cartridge and an electrophotographic apparatus employing the electrophotographic photosensitive member. Particularly, the present invention relates to an electrophotographic photosensitive member which is electrically charged by a charging member placed in contact with it, and to a process cartridge and an electrophotographic apparatus employing the electrophotographic photosensitive member.
2. Related Background Art
Usual electrophotographic processes comprise charging of an electrophotographic photosensitive member, light image exposure of the photosensitive member, development of the formed electrostatic image, transfer of the developed image, fixation of the image, and cleaning of the photosensitive member. Conventionally, the electrical charging of the electrophotographic photosensitive member is conducted by application of a high voltage (DC 5-8 kV) by a metal wire to cause corona charging. This process has disadvantages that the corona products such as ozone and NO.sub.x degenerate the photosensitive member surface to cause blurring and deterioration of the image; and dirt on the wire causes blanks or black streaks in the image. In particular, the electrophotographic photosensitive member comprising an organic photoconductive substance is less stable chemically than inorganic photosensitive members such as a selenium photosensitive member or an amorphous silicon photosensitive member, and is liable to be degenerated by chemical reaction (mainly oxidation) on exposure to the corona products. Therefore, repeated corona charging causes image blurring owing to the degeneration of the photosensitive member, and drop of the copy density owing to the decrease of the sensitivity, resulting in a short printing or copying life of the photosensitive member.
Moreover, in the corona charging, only 5-30% of the entire current is directed to the photosensitive member, and a larger portion of the current flows to the shield plate, which lowers efficiency of the charging means.
To offset the above disadvantages, studies are made to conduct electrical charging with a charging member placed in contact with the electrophotographic photosensitive member without employing corona charger as disclosed in publications of Japanese Patent Application Laid-Open Nos. 57-178267, 56-104351, 58-40566, 58-139156, 58-150975, and so forth. Specifically, a charging member like an electroconductive elastic roller is placed in contact with the photosensitive member, and a DC voltage of about 1-2 kV is applied to the charging member to charge the surface of the photosensitive member at a prescribed potential.
However, the direct charging is liable to cause nonuniform charging, and dielectric breakdown of the photosensitive member.
To solve the above problems to improve the uniformity of the charging, a method is disclosed in which a superposition voltage of an AC voltage (V.sub.AC) and a DC voltage (V.sub.DC) is applied to the charging member (Japanese Patent Application Laid-Open No. 63-149668). In this method, the peak-to-peak voltage (V.sub.pp) of the superposed AC should be twice or more times as high as the absolute value of the DC voltage to obtain the uniform charging effect.
However, with increase of the superposed AC voltage, dielectric breakdown becomes liable to be caused by the maximum voltage of the applied pulse voltage when slight defects are present in the photosensitive member. In particular, this dielectric breakdown is severe in the photosensitive member having a low dielectric strength. In such a case, a white blank can be formed in a positive development system, or a black band can be formed in a reversal development system in a length direction of the contact portion. If a pinhole is present, the current will leak through the pinhole site to drop the voltage applied to the charging member. The amount of the leakage current is so large that the photosensitive member may be damaged greatly as to be abraded and the durability is impaired, disadvantageously.
To solve the above problems, the publication of Japanese Patent Application Laid-Open No. 6-3921 discloses charging (injection charging) to inject electric charges directly to the surface layer of the photosensitive member without electric discharge. This charging method hardly damages the photosensitive member, since the difference between the voltage applied to the charging member and the surface potential of the photosensitive member is very small and ozone is not generated because of the direct injection of electric charge through a charging member to the surface layer (charge injection layer) containing electroconductive particles dispersed therein. However, this charging method has a disadvantage in that the charging tends to be nonuniform since the charge injection is conducted only at the contact points of the charging member.
With the demand for higher image quality in recent years, electrophotographic photosensitive members are being investigated which are capable of forming finer and more uniform picture images.